Because move power is rounded down, many Pokemon see no benefit in Fast Move DPS when levelling from 30-40. This further emphasises the decreasing returns on levelling Pokemon up described in Stardust

Winning the battle might not win the war. A single Cloyster will lose against a Dragonite, and a Lapras will win because of its larger health pool. However, the Lapras will take twice as much damage in doing so. Depending on your Potion/Revive supply ratio, you may prefer to use Cloyster over Lapras.

When facing a tough opponent, you want to be sure your special move will finish them off, otherwise you've just gifted the defender a lot of Energy with which to hit you back with a special move.

Battle Simulator Explained

This battle simulator uses the known in-game formulas to instantly simulate a Pokemon Go battle and determine its outcome. Upon selecting your Pokemon, their possible moves will be displayed for you to select from.

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These white boxes show the moves base DPS after STAB but before factoring in battle specific factors like the Pokemon's Attack, Defense and defending Pokemon's type.

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These pink text boxes show the Damage per Energy (DPE) for charge moves. In many cases, this is more important than the DPS for a charge move. See this discussion on why DPE is so important for charge moves.

These weather icons will appear when you have selected a weather setting that matches the move's type. Move damage will be boosted by the weather damage bonus (120%)

The four sliders allow you to adjust the Pokemon's IVs and level. When sliding these values, you can see the "Base Stats" underneath changing in real time. The base stats for attack, defence and stamina are simply the species' stats found in Pokemon Stats, plus any IV value you have added for your Pokemon. The HP and CP are calculated using the following known formulas:

The "Battle Stats" section then takes the base stats of the Pokemon and move's you have selected and converts them into battle specific stats that take into account the opposing Pokemon's stats. This includes doubling the defender's HP and performing the following operations.

The battle is now ready for simulation. Upon hitting "Battle" the simulation will begin and take place as fast as your computer/phone can calculate it. The battle processes using the following assumptions:

The first attacker move can occur at 99.3 seconds on the battle countdown timer.

The first defender move can occur at 98.4 seconds on the battle countdown timer.

Special moves are cast as soon as the Pokemon has enough energy to do so.

Fast moves are cast back-to-back, except that the defending Pokemon has an addition 2 second pause after each fast attack.